1. Field of the Invention
The present invention relates to a power amplifier, a power amplifying method amplifying high-frequency power, and a radio communication apparatus using the power amplifier.
2. Description of the Prior Art
It is required that for size reduction and power saving, power amplifiers used for portable telephone terminals and base stations have high-power and high-efficiency characteristics. In particular, when wide-band transmission signals such as W-CDMA signals are handled, it is necessary that power amplifiers have a wide band and little distortion.
However, the frequency spectrum of a modulated wave whose carrier wave is modulated by a modulating wave is normally distributed in a band of approximately the frequency of the modulating wave. When a signal of the frequency of the modulated wave having such a frequency spectrum is input to a power amplifier, because of the nonlinearity of an amplifying element such as a FET (field effect transistor) used for the power amplifier, a second-order intermodulation distortion component that appears at a frequency which is the difference between signals of different frequency components of the modulated wave is caused in addition to an intermodulation distortion component.
Moreover, as mentioned above, to provide high-power characteristics, a FET arranged in parallel in a multifinger structure or a multiplicity of FETs combined in parallel to increase the gate width is used as the amplifying element of the power amplifier.
In such power amplifiers, when the impedance at the frequency of the modulating wave of the modulated wave on the output side of the FET is high to a degree, a second-order intermodulation distortion component is caused that appears at the frequency which is the difference between signals of different frequency components of the modulated wave. The frequency of the second-order intermodulation distortion component is distributed in the neighborhood of the frequency of the modulating wave, and the second-order intermodulation distortion component is again mixed with the amplified signal at the drain electrode of the FET to make intermodulation distortion worse. This indicates that the linearity of the FET is not effectively used.
FIG. 10 shows a conventional power amplifier 1113 being excellent in distortion characteristic. The power amplifier 1113 of FIG. 10 comprises an input terminal 1101, matching circuits 1102, 1106 and 1110, a FET 1103, inductors 1104 and 1109, capacitors 1105 and 1108, a quarter-wave stripline 1107, an output terminal 1111, and a bias supply power source terminal 1112.
The matching circuit 1102 is a circuit that matches the impedance of the input terminal 1101 to that on the drain side of the FET 1103.
The inductor 1104 and the capacitor 1105 are circuits that serially resonate at the frequency of the modulated wave. It is assumed that the frequency of the modulated wave is, for example, 1 GHz and the frequency of the modulating wave of the modulated wave is, for example, 20 MHz.
The matching circuit 1106 is a circuit that matches the impedance on the output side of the FET 1103 to that on side of the matching circuit 1110.
The double wave shorting circuit 1107 is a circuit that is short-circuited for the harmonic of the frequency of the modulated wave, for example, a stripline.
The capacitor 1108, the inductor 1109 and the bias supply terminal 1112 constitute a bias choke circuit that supplies a bias voltage to the gate of the FET 1103.
The matching circuit 1110 is a circuit that matches the impedance on the side of the matching circuit 1106 to that on the side of the output terminal 1111.
Next, the operation of the conventional power amplifier will be described.
Since the signal of the frequency of the modulated wave (1 GHz) is modulated by the modulating wave (20 MHz) of the modulated wave as mentioned above, the frequency of the signal of the frequency of the modulated wave is distributed, for example, in a band of approximately ±20 MHz from the neighborhood of 1 GHz.
When input to the input terminal 1101, the signal of the frequency of the modulated wave has its impedance matched by the matching circuit 1102 and is input to the gate of the FET 1103. The drain of the FET 1103 is supplied with a bias voltage by the bias choke circuit constituted by the bias supply terminal 1112, the capacitor 1108 and the inductor 1109.
Consequently, the signal of the frequency of the modulated wave input to the gate of the FET 1103 is power-amplified by the FET 1103, and is output from the drain of the FET 1103 as an amplified signal. Because of the non-linearity of the FET 1103, the amplified signal also includes a second-order intermodulation distortion component that appears at a frequency which is the difference between signals of different frequency components of the modulated wave. The second-order intermodulation distortion component is distributed in the neighborhood of the frequency of the modulating wave (20 MHz).
The constant of a resonance circuit constituted by the inductor 1104 and the capacitor 1105 is set so that the resonance circuit serially resonates in the neighborhood of the frequency of the modulating wave. Therefore, the impedance thereof is short-circuited at the frequency of the modulating wave (20 MHz) and is high at the frequency of the modulated wave (1 GHz).
Consequently, since the second-order intermodulation distortion component included in the amplified component is short-circuited by the resonance circuit constituted by the inductor 1104 and the capacitor 1105, the signal component that varies according to the frequency of the modulating wave is reduced in the voltage on the drain side of the FET 1103. Consequently, the above-mentioned problem is reduced that the second-order intermodulation distortion component is mixed with the amplified signal at the drain to make intermodulation distortion worse.
The amplified signal output from the FET 1103 has its second-order intermodulation distortion component smoothed by the inductor 1104 and the capacitor 1105 as mentioned above, and has its impedance matched by the matching circuit 1106. The circuit constituted by the double wave shorting circuit 1107 and the capacitor 1108 is short-circuited by the high-order harmonic (signal having frequency spectra of approximately 2 GHz and not less than 2 GHz) of the modulated wave. Therefore, the high-order harmonic of the modulated wave included in the amplified signal output from the matching circuit 1106 is short-circuited by the circuit constituted by the double wave shorting circuit 1107 and the capacitor 1108. The amplified signal having its high-order harmonic thus reduced has its impedance matched by the matching circuit 1110, and is output from the output terminal 1111.
As described above, it is necessary that power amplifiers used for a communication mode such as W-CDMA have a wide band and little distortion. To achieve this, it is extremely important that the impedance of the circuit part on the output side of the amplifying element at the frequency of the modulating wave (20 MHz) be lower than the impedance on the output side of the amplifying element at the frequency (1 GHz) of the modulated wave as mentioned above.
Moreover, if the impedance of the circuit part on the output side can be reduced in a structure other than the structure used by the conventional power amplifier 1113 described with reference to FIG. 10, the degree of freedom of design will improve accordingly.
That is, a power amplifier is required in which the impedance of the circuit part on the output side of the amplifying element at the frequency of the modulating wave can be reduced in a structure different from that of the conventional power amplifier.
Moreover, while in the conventional power amplifier 1113 described with reference to FIG. 10, the impedance at the frequency of the modulating wave (20 MHz) is made close to short-circuiting by the resonator constituted by the inductor 1104 and the capacitor 1105, in actuality, since some loss is caused at the frequency of the modulating wave at the inductor 1104 and the capacitor 1105, it is difficult to create an ideal short-circuiting at the frequency of the modulating wave. To reduce the loss, it is necessary to increase the physical sizes of the inductor 1104 and the capacitor 1105, and this increases the size of the power amplifier.
That is, it is difficult that the impedance of the circuit part on the output side of the amplifying element at the frequency of the modulating wave is lower, and consequently, it is difficult to more effectively use the linearity of the amplifying element.
In radio communication systems preceding a communication mode such as W-CDMA, since the frequency band used for communication is narrow, such a problem did not arise. However, in recent wide-band systems such as W-CDMA systems, it is an increasingly important problem.